This happens sometimes when you are working in AS. AS will open with the program that was opened last. For example if you opened Mechanical Desktop closed it then opened AS then AS will open with Mechanical Desktop in the background. The same is true when you open a version of AutoCAD for example, you have AS 2015 installed and you also have AutoCAD 2014. You open AutoCAD 2014then close it; the next time you open up AS 2015 it will open on AutoCAD 2014.
